为什么两个程序有前向引用错误而第三个没有？ [英] Why do two programs have forward referencing errors while the third does not?

问题描述

以下内容无法编译，提供非法转发引用消息：

The following does not compile, giving an 'illegal forward reference' message:

class StaticInitialisation {

    static
    {
        System.out.println("Test string is: " + testString);
    }

    private static String testString;

    public static void main(String args[]) {
        new StaticInitialisation();
    }
}

但是，以下编译：

class InstanceInitialisation1 {

    {
        System.out.println("Test string is: " + this.testString);
    }

    private String testString;

    public static void main(String args[]) {
        new InstanceInitialisation1();
    }
}

但以下不编译，给出'非法转发引用'消息：

But the following does not compile, giving an 'illegal forward reference' message:

class InstanceInitialisation2 {

        private String testString1;

    {
        testString1 = testString2;
    }

    private String testString2;

    public static void main(String args[]) {
        new InstanceInitialisation2();
    }
}

为什么StaticInitialisation和InstanceInitialisation2不能编译，而InstanceInitialisation1会这样做？

Why do StaticInitialisation and InstanceInitialisation2 not compile, while InstanceInitialisation1 does?

推荐答案

这部分由 8.3.3 ：

使用类变量，其声明在使用后以文本形式出现有时受到限制，即使这些类变量在范围内（第6.3节）。具体来说，如果满足以下所有条件，则为编译时错误：

Use of class variables whose declarations appear textually after the use is sometimes restricted, even though these class variables are in scope (§6.3). Specifically, it is a compile-time error if all of the following are true:

• 声明类变量类或接口C在使用类变量后以文本方式显示;

• The declaration of a class variable in a class or interface C appears textually after a use of the class variable;

在C的类变量初始化程序或静态初始化程序中使用是一个简单名称of C;

The use is a simple name in either a class variable initializer of C or a static initializer of C;

使用不在作业的左侧;

The use is not on the left hand side of an assignment;

C是封闭使用的最内层类或接口。

C is the innermost class or interface enclosing the use.

使用实例变量，其声明在文本后出现在即使这些实例变量在范围内，有时也会限制使用。具体来说，如果满足以下所有条件，则为编译时错误：

Use of instance variables whose declarations appear textually after the use is sometimes restricted, even though these instance variables are in scope. Specifically, it is a compile-time error if all of the following are true:

• 在a中声明实例变量在使用实例变量后，类或接口C以文本方式显示;

• The declaration of an instance variable in a class or interface C appears textually after a use of the instance variable;

在C的实例变量初始化程序或实例初始化程序中使用是一个简单名称of C;

The use is a simple name in either an instance variable initializer of C or an instance initializer of C;

使用不在作业的左侧;

The use is not on the left hand side of an assignment;

C是封闭使用的最里面的类或接口。

C is the innermost class or interface enclosing the use.

在你的第二种情况，使用不是一个简单的名称 - 你明确地得到了这个。这意味着它不符合上面引用的第二个列表中的第二个项目符号，因此没有错误。

In your second case, the use isn't a simple name - you've got this explicitly. That means it doesn't comply with the second bullet in the second list quoted above, so there's no error.

如果您将其更改为：

System.out.println("Test string is: " + testString);

...然后它将无法编译。

... then it won't compile.

或者在相反的方向，您可以将静态初始化程序块中的代码更改为：

Or in the opposite direction, you can change the code in the static initializer block to:

System.out.println("Test string is: " + StaticInitialisation.testString);

奇怪，但这就是它的方式。

Odd, but that's the way it goes.

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